This is a double-blind, placebo-controlled efficacy and safety evaluation of sublingual immunotherapy (SLIT) at two different dosing regimens in patients with house dust mite-induced allergic rhinitis/mild intermittent asthma. The study will examine two dosing regimens of SLIT to determine the optimal dose for a larger scale clinical trial as a potential treatment for preventing the development of childhood asthma. It will determine the safety and efficacy of a European-based approach versus a dosing of immunotherapy as practiced by proponents of SLIT in the United States in comparison with placebo. [unreadable] Eligible participants in the study will be randomized to receive either placebo, a low dose SLIT therapy and a higher dose SLIT therapy. Treatment will be conducted over a period of 18 months. Participants will be evaluated over the course of 18 months after the initiation of treatment to determine the effectiveness of the regimens. Both clinical efficacy and safety will be examined and potential immunologic mechanisms will be evaluated; both in vitro and in vivo. The in vitro tests will be done to examine whether SLIT is effective in shifting the house dust mite-induced immune response away from a Th2 paradigm and toward an immune tolerance state and whether SLIT acts via local mucosal versus systemic immunological effects. The in vivo studies will examine whether subjects will have a prolonged right order shift in the early and late phase responses to SLIT-induced allergy skin test reactions and bronchial allergen challenges as measured by bronchoprovocation after therapy. Lastly, we will monitor and assess the tolerability and safety of SLIT in the treatment of allergic disease. [unreadable] [unreadable]